When a wireless base station communicates with a mobile station (or a mobile terminal) in downlink of a mobile communications system, the transport format, such as the modulation scheme or the coding rate, may be adjusted in an adaptive manner according to the radio conditions of the downlink channel. This type of control is called adaptive modulation and coding (AMC).
In AMC, a mobile station monitors the radio condition of the downlink, and reports the downlink radio condition to the base station in uplink. The downlink radio condition includes, for example, the signal to interference ratio (SIR), the carrier to interference ratio (CIR), and receive power.
The wireless base station determines the transport format, such as the modulation scheme and the coding rate, for downlink communication, based on the downlink radio condition reported from the mobile station and the available radio resources in downlink in order to perform signal transmission according to the downlink radio condition. If the radio condition is good because the mobile station is located near the wireless base station or because the traveling speed of the mobile station is slow, a transport format that allows higher rate data transmission is selected. In contrast, under unsatisfactory radio conditions, a transport format suitable to lower rate transmission is selected. In this manner, wireless communications can be performed according to change in the radio environment.
The AMC technique is employed in so-called high speed downlink packet access (HSDPA) in the third generation mobile communications systems based on the IMT-2000 standard.
HSDPA is a new communication scheme supported by the 3GPP Release 5 standard, and the data rate is improved by a factor of 5 or more, as compared with the conventional W-CDMA. See 3GPP TS25.214 v5.8.0 and 3GPP TR25.848 v4.0.0. From the same point of view, the 3GPP2 supports the 1x-EVDO (1x-Evolution Data Only) standard dedicated for downlink high-rate data transmission. See 3GPP2 C.S0024-A v1.0.
The adaptive modulation and coding technique employed in HSDPA controls the modulation scheme or the coding rate of the radio channel according to the radio condition between the base station and the mobile station. The control information (radio quality information) transmitted from the mobile station to report the downlink radio condition is called a channel quality indicator (CQI), and this control information is mapped to a dedicated uplink control channel HS-DPCCH adapted for HSDPA.
In general, a mobile station estimates the channel quality indicator (CQI) based on the signal to interference ratio determined from the symbols of the downlink common pilot channel (CPICH). The CQI is estimated such that the transport block error rate (BLER) becomes 10% when receiving the transport format determined from that CQI.
FIG. 1 is a schematic block diagram of the MAC-hs processing unit of a conventional wireless base station. “MAC-hs” is the abbreviation for Media Access Control-HSDPA. In the MAC-hs processing unit 1112, a CQI is decoded at the layer-1 processing unit 1111. The decoded CQI is supplied to the CQI acquiring unit 1110, and then to the TFR (transport format and resource) selection unit 1170. The MAC-hs resource calculation unit 1120 includes an HS-DSCH power resource calculation unit 1121 and an HS-DSCH code resource calculation unit 1122 to calculate radio resources (such as power resources, code resources, or hardware resources) being allocated to the high-speed downlink shared channel (HS-DSCH).
The TFR selection unit 1170 determines the downlink transport format, including the modulation scheme, the number of codes, the transport block size (TBS) and the power offset, based on the channel quality indicator (CQI) received from the CQI acquiring unit 1110 and the radio resources (the power resources and the code resource) calculated by the MAC-hs resource calculation unit 1120 for allocation to the HS-DSCH. The downlink transport format and transmit power level determined by the TFR selection unit 1170 are supplied to the layer-1 processing unit 1111. The layer-1 processing unit 1111 performs downlink signal transmission using the transport format determined by the TFR selection unit.
In HSDPA, hybrid automatic repeat request (H-ARQ) control is also performed between the base station and the mobile station. The mobile station receives a downlink, maps the decoding result (OK or NG) of the received data to the HS-DPCCH, and transmits it in uplink as a confirmation message of the downlink transmission. The confirmation message includes ACK, NACK, and DTX. The ACK message represents the safe receipt of the data, the NACK message represents negative acknowledge, and the DTX message represents that the downlink itself has not been received because the mobile station cannot receive the downlink shared control channel HS-SCCH for some reason. Based on the confirmation message from the mobile station, the base station retransmits the data in the H-ARQ control.
In adaptive modulation and coding for determining the downlink transport format at the base station according to the downlink radio quality reported from the mobile station, it takes at least 5 ms to 7 ms from the measurement of the downlink radio condition at the mobile station up to the actual signal transmission from the base station based on the measurement result. Accordingly, if the propagation environment varies frequently, the adaptive modulation and coding cannot follow the change in the radio environment. If the traveling speed of the mobile station is fast, and if the adaptive modulation and coding is behind the change in the transmission channel, then the throughput of the downlink is degraded.
Another problem is the accuracy of the CQI estimation. The mobile station calculates the CQI such that the downlink block error rate will become 10%. However, if the propagation environment changes too quickly, the CQI cannot be calculated accurately, and may not bring the downlink block error rate to or near 10%. Then the base station cannot determine the transport format appropriately because of the inaccurate CQI.
In addition, since the mobile station calculates the CQI based on the required block error rate, it is difficult for the base station to control the block error rate. If there are multiple mobile stations under the base station, the actual block error rate varies among the mobile stations due to differences or variations in performance.